It is common practice in a ship's engine room to place an isolation valve on the exhaust side of a steam turbine and upstream of other steam operated equipment. A valve commonly used for this purpose will include a housing or casing normally having a single inlet and two outlets with valve closures upstream of each outlet. One of these closures is identified as a main valve, and is usually handwheel operated with spring loading and adapted to be opened against force of the spring to dump steam to the exhaust system. This main valve includes a disc which is adapted to open downstream due to a greater pressure upstream, much in the same manner as a relief valve. The disc is opened by a stem withdrawn by rotation of the handwheel. This is the normal open operating position. There is a disadvantage to this arrangement in that turning down the stem too far positively prevents the main valve from opening, even as a relief valve, regardless of upstream pressure, thus creating an operating hazard. The other closure is a secondary valve which may be provided for discharging steam to the atmosphere. It is usually a wrench operated valve, open only when the turbine is out of service, and is used to prevent pressurization of the valve casing due to leakage through the steam turbine. Another disadvantage of the prior arrangement is that leaving the auxiliary valve open prevents opening of the main valve. Such an isolation valve is illustrated in FIG. 1.